Based on historical cash flows into the two funds, and market performance through Tuesday, the newsletter pegs assets for the Vanguard 500 Index (VFINX) at $107.2 billion and $106.9 billion for Fidelity Magellan (FMAGX). The clincher may have been Magellan's one-day performance loss of 1.5 percent on Tuesday, vs. the S&P 500 Index
SPX, +0.01%
loss of 0.7 percent for the day.

Assets of the two funds have been under intense scrutiny in recent months.

"The question has never been if Index 500 would pass Magellan," Editor Daniel P. Wiener said in a statement. "The question was when."

Vanguard's flagship fund is open to all investors while Fidelity's Magellan is only available to current investors or through retirement accounts.

If true, Vanguard's new leadership position is not a fait accompli. Investors should expect to see some give and take over the coming months, in part due to Magellan manager Bob Stansky's ability to outperform the Vanguard Index 500 fund, according to Wiener -- who also noted an unusually large outflow from the Vanguard flagship fund last month, to the tune of $750 million.

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